1. Field of the Invention
The present invention generally relates to a semiconductor integrated circuit. In particular, the present invention generally relates to a frequency synthesizer and a frequency synthesizing method.
2. Description of the Related Art
With the increasing demand for information communications, the mobile communication system market is expanding rapidly. Thus, a great deal of research is being done in relation to low-cost, low-power consumption, and small-volume systems. As a result, Complementary Metal Oxide Semiconductor (CMOS) process technology, and semiconductor devices using a small-size chip and operating at a high-frequency have been introduced.
A high-frequency, low-noise, and low-power phase-locked loop (PLL) may be used in various fields such as optical data links and asynchronous transfer mode (ATM) systems. The PLL normally has a high operating frequency, a short response time, small clock jitter, a wide input locking range, and a linear voltage-to-frequency conversion characteristic. These characteristics of the PLL serve as factors in determining the performance of a voltage controlled oscillator (VCO). In general, the VCO is designed using an LC tank circuit or a ring oscillator.
FIG. 1 illustrates a conventional LC tank circuit according to the Related Art. Referring to FIG. 1, in an LC tank circuit 100, two pairs of an inductor L and a capacitor C are series connected between a supply voltage VDD and a ground voltage VSS. Inductors L and capacitors C connect at nodes 102 and 104, respectively. NMOS transistors MT1 and MT2 are connected between nodes 102 and 104, respectively, and the ground voltage VSS. Gates of transistors MT2 and MT1 are cross-connected to nodes 102 and 104, respectively. In the LC tank circuit 100, an output voltage Vout typically oscillates according to discharging/charging operations of capacitors C.
The LC tank circuit 100 normally has a high Q value and may produce a clear signal due to low phase noise and small clock jitter. However, the LC tank circuit 100 typically has a small tuning range and may need a large layout area to form inductors.
A VCO using a ring oscillator typically may be easily integrated because a ring oscillator typically requires a small layout area. Such a ring-oscillator-type VCO typically has a large operational range. However, the VCO that uses a ring oscillator normally cannot implement high-frequency operation and may undesirably generate large phase noise.